1. Field of the Invention
The present invention relates to a fuel injection system of an internal combustion engine and, more particularly, to an air-assisted fuel injection system for use in an internal combustion engine.
2. Description of the Related Art
FIG. 3 is a schematic illustration of a known air-assisted fuel injection system of an internal combustion engine such as, for example, a spark-ignited gasoline engine. The internal combustion engine, generally designated at A, has an intake pipe 15 which is provided with a throttle valve 1 for controlling the rate of supply of air to the internal combustion engine A. An air flow rate sensor 2 for sensing the flow rate of air is disposed upstream of the throttle valve 1. An air-assisted fuel injector 3 has an end confronting the internal combustion engine A and injects fuel into the engine A at a tale optimized in accordance with information given by the air flow rate sensor 2. An atmospheric air introduction passage 4 is connected at its one end to a portion of the intake pipe 15 downstream of the air flow rate sensor 2 and at its other end to the fuel injector 3. The arrangement is such that assist air is introduced through the atmospheric air introduction passage 4 due to the pressure differential between the interior of an air duct 16 of the intake pipe 15 and an intake manifold 11, so as to collide with the fuel atomized from the fuel injector 3.
In operation, the rate of supply of air into the internal combustion engine A is controlled by opening and closing the throttle valve 1. The rate of supply of the air is sensed by the air flow rate sensor 2. The air flow rate information obtained by the air flow rate sensor 2 is delivered to the computer unit 9 which sets the fuel injection rate to an optimum level. The air-assisted fuel injector 3 then injects fuel at the set fuel injection rate. Meanwhile, assist air is supplied to the fuel injector 3 through the atmospheric air introduction passage 4 due to the pressure differential between the intake manifold pressure inside the intake manifold 11 and the air pressure in the air duct 16, so as to collide with the fuel atomized from the fuel injector 3, thereby promoting atomization and micronization of the fuel.
The described known fuel injection system of internal combustion engine suffers from a problem in that the pressure differential across the air-assisted fuel injector 3 is reduced when the throttle valve 1 has been opened to a large degree i.e., when the intake manifold pressure has approached the atmospheric air pressure, so that the effect of assist air in promoting fuel atomization is reduced undesirably.